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KVM: MMU: Reinstate pte prefetch on invlpg

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Commit fb341f57 removed the pte prefetch on guest invlpg, citing guest races.
However, the SDM is adamant that prefetch is allowed:

  "The processor may create entries in paging-structure caches for
   translations required for prefetches and for accesses that are a
   result of speculative execution that would never actually occur
   in the executed code path."

And, in fact, there was a race in the prefetch code: we picked up the pte
without the mmu lock held, so an older invlpg could install the pte over
a newer invlpg.

Reinstate the prefetch logic, but this time note whether another invlpg has
executed using a counter.  If a race occured, do not install the pte.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Avi Kivity
2010-03-15 13:59:57 +02:00
parent fbc5d139bb
commit 08e850c653
3 changed files with 39 additions and 14 deletions
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37
arch/x86/kvm/mmu.c
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@@ -2613,9 +2613,30 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
int flooded = 0;
int npte;
int r;
int invlpg_counter;
pgprintk("%s: gpa %llx bytes %d\n", __func__, gpa, bytes);
invlpg_counter = atomic_read(&vcpu->kvm->arch.invlpg_counter);
/*
* Assume that the pte write on a page table of the same type
* as the current vcpu paging mode. This is nearly always true
* (might be false while changing modes). Note it is verified later
* by update_pte().
*/
if ((is_pae(vcpu) && bytes == 4) || !new) {
/* Handle a 32-bit guest writing two halves of a 64-bit gpte */
if (is_pae(vcpu)) {
gpa &= ~(gpa_t)7;
bytes = 8;
}
r = kvm_read_guest(vcpu->kvm, gpa, &gentry, min(bytes, 8));
if (r)
gentry = 0;
new = (const u8 *)&gentry;
}
switch (bytes) {
case 4:
gentry = *(const u32 *)new;
@@ -2628,22 +2649,10 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
break;
}
/*
* Assume that the pte write on a page table of the same type
* as the current vcpu paging mode. This is nearly always true
* (might be false while changing modes). Note it is verified later
* by update_pte().
*/
if (is_pae(vcpu) && bytes == 4) {
/* Handle a 32-bit guest writing two halves of a 64-bit gpte */
gpa &= ~(gpa_t)7;
r = kvm_read_guest(vcpu->kvm, gpa, &gentry, 8);
if (r)
gentry = 0;
}
mmu_guess_page_from_pte_write(vcpu, gpa, gentry);
spin_lock(&vcpu->kvm->mmu_lock);
if (atomic_read(&vcpu->kvm->arch.invlpg_counter) != invlpg_counter)
gentry = 0;
kvm_mmu_access_page(vcpu, gfn);
kvm_mmu_free_some_pages(vcpu);
++vcpu->kvm->stat.mmu_pte_write;